Cryopreservation of different cell types is critically influenced by cooling rate and cryoprotectant. Overly fast cooling rates can cause lethal intracellular formation of ice crystals. At the other end of the spectrum, overly slow cooling rates can result in osmotic shock injury to cells. Further, conventional cryoprotectants, while essential to the successful freezing of cells, are often toxic to the same cells in the thawed state.
Research has shown that for most biological cells, there is a specific cooling rate that may be considered optimal for the cell type. For example, for human hematopoietic progenitor cells (HPCs) the optimal cooling rates are known to be from about 1° C./min. to about 3° C./min. Currently utilized commercial devices for cooling cells for cryopreservation accomplish controlled temperature changes by injecting liquid nitrogen vapor through an electromagnetic valve. As the temperature inside the devices increases or decreases, additional liquid nitrogen is injected to maintain the desired cooling rate.
Disadvantageously, these cooling devices are expensive, require high rates of liquid nitrogen vapor consumption, and are unreliable due to the relative fragility of the required electromagnetic valves. Accordingly, there is a need in the art for inexpensive, reliable devices for maintaining narrowly tailored, optimal cooling rates for various cell types to be cryopreserved.
It is desirable to maintain stores of a variety of cell types, some of which may be relatively fragile cells, for future use. For example, platelets, particularly human platelets, are in wide demand for a variety of uses requiring hemostasis such as standard transfusions in case of illness or trauma, chemotherapy, bone marrow transplants, and the like. The most common method for storage of platelets and certain other fragile cell types in, for example, blood banks, is so-called liquid storage, wherein platelets are maintained in liquid solutions at room temperature for a maximum of 5 days, and discarded if not used. Attempting longer-ten storage of platelets under such conditions results in progressive platelet aging and cytokine secretion, and risks microbial contamination. Further, screening processes for ensuring that donor platelets are immunologically compatible with prospective transfusion candidates may require 3-5 days to complete, further shortening the window of usefulness for stored platelets. Finally, there is currently no means for patients to store autogeneic platelets for long periods of time for future use.
Cryogenic preservation of cells is commonly employed when long-term storage of such cells is desirable. However, at present there are no reliable cryopreservation techniques for fragile cell types such as platelets which avoid the problems of loss of, e.g. viability, ability to secrete cytokines, and membrane stability. Accordingly, there is a need in the art for methods and devices for cryopreservation of cells, particularly fragile cells such as platelets and hematopoietic stem cells, which are reliable and do not result in cell damage and losses in viability. The present invention satisfies this need in the art by providing a method for cryopreservation of cells. The invention further provides devices suitable for cryopreservation of cells, including fragile cells, in accordance with the methods described.